1. Field of the Invention
This invention relates to a leaching process for reducing the pyritic sulfur content of coal while the same is being transported in a pipeline in the form of a water slurry. The process preferably is conducted at a temperature of 70.degree. to 100.degree. F at ambient pressure with a concentration of oxidant ranging from 1 to 1.5 times the stoichiometric amount of sulfur estimated present in the coal and with a turbulance ranging from a Reynolds Number of about 2000 to about 3000.
Coals are not burned at the mine-site except in rare instances and as a consequence, the coal must be transported to the point of use. This is done with unit and integral trains, slurry pipelines, trucking and barging on inland waterways. The economics that govern the selection of the transportation mode depend on a variety of factors such as the availability of railroad lines, a waterway, and the distance the coal must be transported.
Coals to be transported may require a pre-treatment or clean-up to remove inert materials as mineral matter and water. More recently, regulations established by the Environmental Protection Agency have set a limit for new stationary plants of 1.2 pounds of SO.sub.2 emissions that may be discharged to the atmosphere per million Btu heat input. As a consequence of this act, the majority of the coals in the United States are removed from the fuel slate of the country unless the sulfur in the coal is reduced.
Sulfur is found in coal in three forms. These are referred to as pyritic sulfur, organic sulfur, and sulfate sulfur. Treating the coal in the presence of hydrogen and hydrogen rich solvents, hydrogen donors, at elevated temperatures and pressures can effectively be used to give a carbonaceous product having a sulfur concentration low enough to meet Federal Specifications. However, the method is expensive and may be a more severe treatment that the coal may require. Thus by merely removing one of the sulfur components of the coal many of the coals can then be burned within the EPA emission limits. This is an approach now under consideration, that is removal or reduction or pyritic type sulfur, ferrous disulfite (FeS.sub.2). FeS.sub.2 occurs in two crystalline forms, one known as pyrite and the other as marcasite. The former is the most common in the United States although mixtures of the two may occur. There are two general methods for removing pyritic sulfur from coal, one relies on the difference in the physical characteristics of the coal and iron pyrite while the other depends on the chemical conversion of the iron pyrite to a water soluble species. Physical separation of the iron pyrite and the coal may be made as the result of gravity difference between the iron pyrite and coal. The former has specific gravities from 4.89 to 5.03 while the range of specific gravities for coal is 1.2 to 1.8. Thus, separations may be made by the use of heavy liquid (dense medium processes) wherein the coal is floated from the pyrite, hydraulic separation by the use of jigs where a particle stratification is achieved as the result of pulsating fluid flow in a bed of particles or by the use of wet concentrating tables, e.g., Diester table, and by flotation that depends on the selective adhesion to air of some solids and the simultaneous adhesion to water of other solids.
Iron pyrite may be selectively oxidized to soluble. sulfates, according to the following reactions: EQU FeS + 4.6 Fe.sub.2 (SO.sub.4) + 4.8 H.sub.2 O .sup. 50.degree.-120.degree. C. .fwdarw.10.2 FeSO.sub.4 + 4.8 H.sub.2 SO.sub.4 + 0.8S EQU feS.sub.2 + 7/2 O.sub.2 + H.sub.2 0 .fwdarw. FeSO.sub.4 + H.sub.2 SO.sub. 4 EQU feS.sub.2 + H.sub.2 0.sub.2 .fwdarw. FeS0.sub.4 .fwdarw. H.sub.2 S0.sub.4 + H.sub.2 0 and etc.